TD-LTE circuit domain fallback multimode single standby terminal implementation
1 Introduction
Although the TD-LTE network is a full packet-switched network, voice services will remain an indispensable and important service for a long time. To ensure the smooth development of high-quality voice services on the TD-LTE network, various standards organizations are actively researching and proposing various voice service solutions.
The possible forms of TD-LTE terminals that can provide voice services are roughly divided into two types, namely multi-mode single standby terminals and multi-mode dual standby terminals. Among them, multi-mode refers to TD-LTE, TD-SCDMA, GSM (GPRS) three modes, multi-mode single standby terminal can use CSFB (circuit domain fallback) or SRVCC (single wireless mode voice call continuity) scheme to provide For voice services, multi-mode dual standby terminals directly use 2G / 3G networks to provide voice services.
This article focuses on several solutions for circuit domain services in the TD-LTE network, especially voice services, explains the technical implementation principles of the TD-LTE circuit domain fallback multimode single standby terminal, and points out the differences in the TD-LTE network During the development period, China's telecom operators can use different technologies to develop voice services.
2 EPS network voice service solution
2.1 Provide voice service (CSFB) based on circuit domain network
CSFB (circuit domain fallback) is one of the achievements of the CS over PS research topic in 3GPP R8. The background proposed by this research topic is that the wireless module of LTE and CS dual-mode terminals is a single wireless mode, that is, dual-mode or multi-mode terminals with LTE and UTRAN / GERAN access capabilities, when using LTE access, they cannot receive / transmit Circuit domain service signal. In order to enable the terminal to initiate CS services such as voice services under LTE access, and to receive paging of voice and other CS services, and to correctly handle the PS services being performed by the terminal in the LTE network, the CSFB technology has been generated.
In the early stage of building a TD-LTE network, if the operator already has a mature UTRAN / GERAN network, in order to protect the CS investment, combined with the deployment strategy of the TD-LTE network, the operator can use the original CS domain voice solution to provide Voice services, while the TD-LTE network only handles data services (including IMS data services). In this case, the CSFB technology is adopted, that is, when the UE under LTE coverage is processing voice services, the terminal first falls back to the CS (circuit domain) network and processes voice services in the CS network; thus, the existing CS domain is reused The purpose of the device is to provide traditional voice services for users in the TD-LTE network.
But the use of CSFB has a prerequisite, that is, only when the overlapping coverage area of ​​E-UTRAN and UTRAN / GERAN, and the user has the CSFB function, can the circuit domain fall back.
The functional architecture of CSFB between LTE and UTRAN / GERAN is shown in Figure 1.
Figure 1 Schematic diagram of CSFB function architecture
In Figure 1, EPC refers to the evolved packet core network.
As can be seen from Figure 1, the key to achieving the CSFB function is that the MSC can establish an SGs interface with the MME in order to realize the joint location update and paging operations of LTE and 2G / 3G. In addition, in order to realize the CSFB function, UE, MME, MSC, EUTRAN, SGSN all need to upgrade to add additional functions.
2.2 Provide voice service based on IMS network (SRVCC)
SRVCC refers to the service continuity of voice calls when a single wireless mode terminal switches from a TD-LTE network to 3GPP UTRAN / GERAN. The SRVCC solution mainly solves the problems that exist when deploying voice services on LTE networks. Under LTE networks, terminals use Voice over LTE based on IMS to establish voice services; when LTE does not reach full network coverage, as users move, ongoing voice The business will face the problem of whether the voice can be continuous after leaving the LTE coverage. At this time, SRVCC can switch the voice to the circuit domain to ensure the uninterrupted voice call.
The SRVCC solution is implemented based on IMS, so the IMS system and SCC AS equipment need to be deployed on the network. In addition, SRVCC will be used in specific scenarios only after the voice service is enabled on the UTRAN / GERAN network.
The network architecture that provides voice services based on the IMS network is shown in Figure 2.
Figure 2 SRVCC network architecture
Figure 2 includes three major modules: EPC, CS and IMS. In order to realize uninterrupted voice calls, three modules are required to work together. Among them, the MSC in the circuit domain needs to be upgraded to an enhanced MSC in order to support the handover process sent from the MME, support the IMS to CS handover and associate CS handover and domain transfer from the IMS domain to the CS domain; Separate the voice and non-voice parts from the PS bearer, initiate SRVCC handover for the voice bearer part and coordinate PS handover and SRVCC handover; HSS in IMS needs to insert the SRVCC VDN (VCC domain transfer number) into the MME during the UE attachment process.
The basic workflow of SRVCC: In order to realize SRVCC, the voice call is anchored in the IMS system. When initiating the domain transfer from E? UTRAN to UTRAN / GERAN, MME (with UE SRVCC related information) first obtains the handover instruction from the E? UTRAN network, and then triggers the SRVCC process to the MSC Server, and the MSC Server initiates the session transfer process to IMS And complete the CS handover procedure to the target cell. After the handover is completed, the MSC Server sends a response (including the necessary handover command information) to the MME and transfers it to the UE for access to UTRAN / GERAN. In this way, the voice call is transferred from E? UTRAN to UTRAN / GERAN.
2.3 Based on multimode dual standby terminal
When the TD-LTE / TD-SCDMA / GSM (GPRS) multi-mode dual standby terminal uses the TD-LTE mode for data services, the terminal can simultaneously call in or call out voice services in the TD-SCDMA / GSM (GPRS) mode; When using the TD-SCDMA mode for data services, the terminal establishes concurrent services under TD-SCDMA to make incoming or outgoing voice services; when using the GSM mode for data services, the terminal uses the GSM mode for incoming voice services or Exhale. The scheme of providing voice services based on multi-mode dual standby terminals does not place special requirements on the network.
3 Multi-mode single standby handheld terminal in the circuit domain
3.1 Working principle
TD-LTE / TD-SCDMA / GSM (GPRS) multi-mode single standby terminal can support independent communication in GSM, TD-SCDMA and TD-LTE modes. Multimode single standby terminal supports TD-LTE, TD-SCDMA and GSM (GPRS) modes, and can only work in TD-LTE mode at any time, TD-SCDMA and GSM (GPRS) mode can only work Send and receive data in the same way as the single-mode terminal in the corresponding mode. The multimode single standby terminal can automatically perform mode conversion between TD-LTE mode, TD-SCDMA, and GSM (GPRS) in the idle state and in the process of circuit domain and packet domain services. In addition, multi-mode single standby terminals can also support reselection and handover between TD-LTE cells and TD-SCDMA, GSM (GPRS) cells.
Generally, multi-mode single-standby terminals use a single card and can support GSM SIM / TD-SCDMA USIM / TD-LTE USIM multi-mode cards. In consideration of providing a more user-friendly experience for existing network users, the terminal should be able to support the SIM card. When the SIM card is used, the TD-LTE multimode single standby terminal is equivalent to an ordinary TD-SCDMA / GSM terminal.
TD-LTE / TD-SCDMA / GSM (GPRS) multi-mode single standby handheld terminal has the ability to access TD-LTE network, TD-SCDMA and GSM (GPRS) network, but cannot use TD-LTE access Circuit domain business. In order to enable the terminal to initiate CS services such as voice under LTE access, to receive paging of CS services such as voice, and to correctly handle the PS services being performed by the terminal under TD-LTE access, the terminal and the network can This problem is solved by CSFB technology. In CSFB technology, in the dual coverage area of ​​TD-LTE and TD-SCDMA / GSM (GPRS), CS services such as dialogue, LCS and supplementary services, the TD-LTE network can trigger the terminal to fall back from TD-LTE access to TD-LTE SCDMA, GSM (GPRS) networks access and carry out CS services. Therefore, CS domain services are provided by TD-SCDMA, GSM (GPRS) networks, and do not require operators to deploy IMS networks. Terminals supporting CSFB do not need to support SRVCC technology.
TD-LTE / TD-SCDMA / GSM (GPRS) multi-mode single standby handheld terminals can initiate or accept CS services (such as voice calls) in the TD-LTE coverage area. When the terminal is calling (MOC), the UE LTE falls back to the CS domain to initiate a call; when the terminal is called (MTC), the UE receives a page in the TD-LTE network and falls back to the CS domain network for subsequent call flow.
For multi-mode terminals supporting CSFB technology, for the TD-LTE packet domain service that the user has established, depending on the operator's strategy and network capabilities, there are two processing methods:
(1) Suspend mode. When the terminal falls back to the circuit domain to perform voice services, the TD-LTE packet domain connection is set to the suspended state. When the terminal ends the voice service and returns to the TD-LTE network, the terminal may request the network to resume the suspended packet domain connection.
(2) With the switching mode. While performing circuit domain service rollback, the terminal can perform PS domain handover and switch ongoing services in the TD-LTE network to the 2G / 3G packet domain to continue execution.
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